1. Field of the Invention
The present invention relates to a method of removing an organic material used on a semiconductor device, and particularly the present invention relates to an ashing method of removing an organic film temporarily formed on a substrate of a semiconductor device during fabrication.
An organic film, such as a resist or a polyamide film temporarily formed on a substrate, which is a part of a semiconductor device, as part of the process of fabricating the semiconductor device has in the past been removed by an ashing method using an oxygen plasma. Removing the resist film is an important part of the process of fabricating a semiconductor device. Removing the resist film, as an organic film, will be described hereinafter. Since the semiconductor device is very small as compared to a Large Scale Integrated circuit device (LSI) or a Very Large Scale Integrated circuit device (VLSI) in which it is used, the resist film, which will be called simply the "resist" hereinafter, is hard to remove, by the usual ashing method using the oxygen plasma, without damaging the devices. During the process of ion implantation and dry etching, which are widely used in process of fabricating process of LSIs or VLSIs, the properties of the resist are substantially changed, causing the ashing rate of the resist to be slow, so that a long time is required to ash the resist. Since resists are used many times in the process of fabricating LSIs or VLSIs, the ashing rate for each resist should be high in order to improve the throughput of the fabrication process.
A layer, called simply a "ground layer" hereinafter, upon which the resist is formed, is usually made of material, such as silicon dioxide (SiO.sub.2 ), polysilicon (Si) or aluminum (Al). Generally, the resist is not easily etched intentionally by the ashing method. In other words, the resist is hard to etch precisely so as to be able to remove only the resist without damaging any of the ground layer in the ashing process. Therefore, when an ashing method is used in the process of fabricating LSIs or VLSIs, great attention must be paid to leaving the ground layer as it is, because the ground layer of the LSI or the VLSI is very thin and not even a small part of it can be permitted to be etched.
There are many kinds of plasma ashing methods for removing a resist film which has been provided on an insulating layer in a semiconductor. The most widely used is a down-flow ashing method because, applying the down-flow ashing method to the plasma ashing process, damage caused by charged particles can be avoided. The down-flow ashing rate generally depends on the temperature, which will hereinafter be called the "ashing temperature", of the resist, such that the ashing rate decreases with a decrease in the ashing temperature. The ashing rate is usually expressed by the well known Arrhenius plot by which the ashing rates are plotted in a line against the inverse numbers of the respective ashing temperatures. In the Arrhenius plot, the gradient of the line gives the activation energy for ashing such that, when the ashing rate decreases rapidly with the decrease of the ashing temperature, the activation energy is large, and such that, when the ashing rate changes only little with the decrease of the ashing temperature, the activation energy is small. In this process, a small activation energy is desirable because ashing can then be performed almost independently of the ashing temperature. In other words, where the activation energy is small, ashing can be performed in a stable and precise manner
Recently, there has been a tendency to use a process for fabricating semiconductor devices performed at lower temperatures, in accordance with the trend of miniaturization of devices as in the LSI or the VLSI. It is desirable to perform the ashing process at a temperature lower than 300.degree. C., most preferably lower than 200.degree. C., to avoid contamination from the resist. In order to maintain a high ashing rate at such low temperatures, higher than 0.5.mu./min for practical use, the activation energy of the ashing rate must also be low. The activation energy of the ashing rate can be changed to some extent by changing the particular reactant gases used for ashing. The selection of the reactant gases, and particularly the use of combinations of such gases, is very important in order to provide high rates and low activation energies, and to precisely etch the resist, while leaving the ground layer as unchanged as possible and for minimizing damage. The selection and the combination of the reactant gases has been studied energetically.
2. Description of the Related Art
Downflow ashing is performed in a downflow of microwave plasma using a microwave plasma resist stripper. This is fully disclosed in a paper titled "Heavy Metal Contamination From Resists during Plasma Stripping" by Shuzo Fujimura and Hiroshi Yano, in Elect. Chem. Soc. Vol. 135, No. 5, May 1988.
The downflow microwave plasma resist stripper comprises a vacuum chamber including a plasma generating chamber, a vacuum pump for exhausting gas in the vacuum chamber, a process chamber including a pedestal on which a sample wafer is placed and a microwave power source. A reactant gas is supplied to the process chamber through the plasma generating chamber.
Then, a reactant gas plasma is generated in the plasma generating chamber by microwaves, so that active species for ashing in the gas plasma proceed to flow down to the process chamber and react with a resist, which has previously been formed on the sample wafer, so as to remove the resist.
In the downflow ashing process, oxygen has long been used as the reactant gas as described before. However, when only oxygen is used, the ashing rate is low and the activation energy is high, so that downflow ashing, using only oxygen, was hard to apply to the process of fabricating LSIs or VLSIs. Therefore, many other reactant gases have been studied as a means to increase the ashing rate and decrease the activation energy. It has even been studied to combine other kinds of gases with oxygen. As a result, several kinds of effective reactant gases have been found as will be described below, giving four examples, a first, a second, a third and a fourth examples, tracing the development of these new reactant gases. Hereinafter, the ashing rate and the activation energy are shown to be related to removing the resist film provided on a semiconductor device by the plasma ashing method.
A first reactant gas was a mixed gas of oxygen (O.sub.2) with a halogenide gas, such as tetrafluoromethane (CF.sub.4). The first reactant gas was most commonly used because it had a high ashing rate. FIG. 1 shows the ashing rate for commercially available photoresist (OFPR-800, TOKYO-OHKA) plotted against the variation of proportion of tetrafluoromethane in the mixed gas, as measured by the flow rate of tetrafluoromethane to the mixed gas at room temperature. Hereinafter, the ashing rate in the case using a particular reactant gas is simply called the ashing rate with the reactant gas. In FIG. 1, when as much as 15% tetrafluoromethane is added to oxygen, the ashing rate reaches a maximum value of 1.5 .mu.m/min at 25.degree. C., which is high enough for practical use. However, the ground layer, such as SiO.sub.2, polysilicon (Si) or Al, is also etched because of fluorine (F) which is found mixed in the first reactant gas. On the other hand, when this first reactant gas is used, the activation energy is drastically reduced to a value of 0.1 eV from 0.52 eV, which is about the same activation energy found when using only oxygen. Such large decrease of the activation energy is due to the tetrafluoromethane, which was explained in the paper, J. J. Hannon and J. M. Cook, J. Electrochem. Soc., Vol. 131, No. 5, pp 1164 (1984).
A second reactant gas was a mixed gas of oxygen and nitrogen (N.sub.2), not containing fluorine (F), which did not etch the ground layer. The ashing rate and the concentration of oxygen atom in the a down-flowed gas were measured by varying the flow ratio of nitrogen to the second reactant gas as shown in FIG. 2; wherein, the concentration of oxygen atom was measured by an actinometry method. In this case, the ashing temperature was 200.degree. C. and the flow rate of the second reactant gas was 1000 Standard Cubic Centimeters per Minute (SCCM). In FIG. 2, white circles represent the concentrations of oxygen atom, obtained from the spectral intensity ratio of the radiation from an oxygen atom (at a wavelength of 6158 .ANG.) to the radiation from an argon atom (at a wavelength of 7067 .ANG.), and triangles represent the concentrations of the same oxygen atom, obtained from the spectral intensity ratio of the radiation from an oxygen atom (at a wave length of 4368 .ANG.) to the radiation from an argon atom (at a wavelength of 7067 .ANG.). Further, the values of these concentrations are normalized by a maximum of the values of the concentrations, positioned at about 10% of the flow ratio of nitrogen to the second reactant gas. Multiplication signs represent the ashing rates at the respective flow ratios of nitrogen to the second reactant gas. As can be seen from FIG. 2, the curve of the ashing rate and that of the concentration of oxygen atom coincide with each other, which means that oxygen atoms are only effective in performing the ashing. FIG. 3 shows an Arrhenius plot of the ashing rate when the second reactant gas contains 90% of oxygen and 10% of nitrogen in the mixture and an Arrhenius plot of the ashing as accomplished with oxygen gas only. The ashing temperature is denoted by T. The ashing rate of the second reactant gas is plotted by a circle and the ashing rate of the oxygen is plotted by a multiplication sign. The ashing rate with the second reactant gas is about two times of that of oxygen alone. Hereinafter, the activation energy of ashing, in the case of using a reactant gas, is simply called the activation energy of the reactant gas. The activation energy (Ea) of the second reactant gas, and that of oxygen are equally 0.52 eV. That is, the activation energy did not change by mixing nitrogen with oxygen. The ashing rate of the second reactant gas, of 0.2 .mu.m/min at 160.degree. C., is too small for practical use. In order to increase the ashing rate, another kind of gas was needed.
The third reactant gas was a mixed gas of oxygen and water vapor (H.sub.2 O) which did not etch the ground layer. The ashing rate and concentration of oxygen atoms were measured by varying the flow ratios of water vapor to the third reactant gas as shown in FIG. 4. The measurements were performed at 180.degree. C. ashing temperature and 1000 SCCM flow rate of the third reactant gas. Circles and multiplication signs in FIG. 4 represent the same as in FIG. 2, respectively. When the flow ratio of water vapor to the third reactant gas exceeded 40%, the concentration of oxygen atoms decreased with an increase of the water vapor flow ratio. However, the ashing rate did not decrease as much as the decrease of the concentration of oxygen atom, as seen in FIG. 4. This means that some active species, other than oxygen atoms, are possibly taking part in the ashing. FIG. 5 compares the Arrhenius plot of the third reactant gas, containing 60% of oxygen and 40% of water vapor, and Arrhenius plot of oxygen gas alone. The ashing rate of the third reactant gas, having 40% flow ratio of water vapor, is plotted by triangles and the ashing rate of the oxygen alone is plotted by multiplication signs. The activation energy of the third reactive gas is 0.39 eV, which is about three quarters of the activation energy (0.52 eV) of oxygen alone, as shown in FIG. 6. FIG. 6 shows the activation energy of ashing in the case of using a third reactant gas by varying the flow ratios of water vapor to the third reactant gas, by white circles. In FIG. 6, the activation energy of ashing in the case using a mixed gas of oxygen and hydrogen, by varying the flow ratio of hydrogen to the mixed gas, is shown by solid circles for the sake of comparison. FIG. 6 shows that the activation energy is easily reduced by adding a little water vapor and the activation energy is constant independent of the flow ratio of water vapor when the flow ratio of water vapor to oxygen exceeds 5%. The activation energy of the second reactant gas is also indicated by a dot chain line in FIG. 6 for comparison with the third reactant gas. It is seen from this comparison that the activation energy does not change by adding nitrogen to oxygen. Behavior similar to the mixed gas of oxygen and water vapor is seen for the mixed gas of oxygen and hydrogen. On the other hand, the ashing rate of the third reactant gas is about 0.22 .mu.m/min at 160.degree. C., as seen in FIG. 5. It has been concluded that the value of the ashing rate of the third reactant gas is still too small for practical use.
The fourth reactant gas is a mixed gas of oxygen, nitrogen and tetrafluoromethane. The fourth reactant gas is disclosed in the Japanese laid-open patent application, SHO 63-102232, titled "DRY ETCHING APPARATUS" by Mikio Nonaka. When the flow ratio of tetrafluoromethane and nitrogen are in the range of 5 to 20% and 5 to 10% respectively, a large ratio of the rate of etching a positive resist to the rate of etching a ground layer is obtained without decreasing the ashing rate. However, the etching of ground layer cannot be avoided in this case.
A mixed gas made by adding as little as 0.2% of hydrogen to a mixed gas of oxygen, nitrogen and tetrafluoromethane is commercially available from EMERGENT TECHNOLOGIES CO. (Phoenix 2320 NORD Photoresist Stripper). In this case, the hydrogen diluted by nitrogen is added in order to improve matching with microwave power. That is, adding hydrogen to the mixed gas is not for reduction of the activation energy. So, the mixed gas is essentially the same as the first reactant gas. In fact, it is also known that the activation energy of the second reactant gas mixed with hydrogen does not decrease until there has been added about 0.5% of hydrogen.
As seen from the description of the first, second, third and fourth reactant gases, an ideal mixed gas, having a high ashing rate and low activation energy and never etching the ground layer, has not been found although much has been studied on new reactant gases.